Ok I am trying to figure out the pros and cons of VBAT and WKP in moving my design from the core to the photon.
In my current design I collect sensor data, transmit then enter DEEP_SLEEP (standby mode) for a long duration to save as much battery as possible. I wakeup either by the IWDG expiration that was set when entering sleep or via an external hardware reset to the RST pin and upon wakeup I have no memory of the past.
So I am looking at the STM32F205RGY6 Datasheet and trying to figure this out. Here’s 2 excerpts
3.18 Low-power modes
• Standby mode
The Standby mode is used to achieve the lowest power consumption. The internal
voltage regulator is switched off so that the entire 1.2 V domain is powered off. The
PLL, the HSI RC and the HSE crystal oscillators are also switched off. After entering
Standby mode, the SRAM and register contents are lost except for registers in the
backup domain and the backup SRAM when selected.
The device exits the Standby mode when an external reset (NRST pin), an IWDG reset,
a rising edge on the WKUP pin, or an RTC alarm / wakeup / tamper /time stamp event
3.19 VBAT operation
The VBAT pin allows to power the device VBAT domain from an external battery or an
VBAT operation is activated when VDD is not present.
The VBAT pin supplies the RTC, the backup registers and the backup SRAM.
Note: When the microcontroller is supplied from VBAT, external interrupts and RTC alarm/events
do not exit it from VBAT operation.
So If I move over to the WKP rather than External Reset via RST so that I can externally wake the device I get what advantage? Is it the the External Reset will wipe the backup registers and the WKP will not?
If I connect a button battery to VBAT does this stop WKP from being able to wake the device? Is the only advantage of a separate battery on VBAT is that the RTC still retains the time? The datasheet says that the backup registers can be preserved in Standby mode if selected.
Any help to clarify the functionality would be most appreciated.